Compounds and compositions containing plutonium



United States Patc 3,000,695 COMPOUNDS AND COMPOSITIONS CONTAININGPLUTONIUM Glenn T. Seaborg, Chicago, Ill., assignor to the Umte Statesof America as represented by the United States Atomic Energy CommissionN Drawing. Filed Dec. 97, 1945, Ser. No. 637,485

44 Claims. (Cl. 23-145) This invention relates to a new chemical elementof atomic number 94, to novel compounds and compositions thereof, and tomethods for their reduction and oxidation.

The term element 94 is used throughout this specification to designatethe element having atomic number 94. Element 94 is also referred to inthis specification as plutonium, symbol Pu. Likewise, element 93 meansthe element having atomic number 93, which is also referred to asneptunium, symbol Np. Reference herein to any of the elements is to beunderstood as denoting the element generically, whether in its freestate, or in the form of a compound, unless otherwise indicated by thecontext.

The apparent discovery of transuranic elements was first announced byEnrico Fermi in 1934. At that time, Fernn' stated that the bombardmentof uranium with neutrons gave beta activities which he attributed totransuranic elements of atomic number 93 and possibly higher. From 1934to 1938 other workers, notably Hahn and Curie extended this work. But in1939, Hahn discovered that the elements which he and others had believedto be transuranic elements were in fact radioactive elements ofintermediate atomic weights produced by the fission of uranium. Hahnsresults were subsequently confirmed, and a great many other fissionproducts in addition to those first found by Hahn were discovered andidentified. Such products were all of lower atomic weight than uranium,generally of atomic numbers in the middle of the periodic system.

So far as is known, prior to about June 1940, no positive evidence wasfound indicating the existence of any transuranic element. However, inJune 1940, E. McMillan and P. Abelson published in the Physical Review,57, 1185 (1940) their discovery that a 2.3 day activity produced by thebombardment of uranium with neutrons was an isotope of element 93,probably 93*. Although it was assumed that the initial product of thebeta decay of the 93 isotope of 2.3 day half-life would be a nucleus ofatomic number 94, there was no proof that any such 94 nucleus could havemore than an ephemeral existence before undergoing spontaneousdisintegration. McMillan and Abelson found no evidence of the productionof any daughter product from their 93 isotope, and, in fact did not evenobtain the 93 isotope itself in pure or useful form.

One phase of the present invention which is especially useful inplutonium recovery processes relates to methods for the control of thestate of oxidation of plutonium. An object of this phase of theinvention is to provide means for attaining a plurality of oxidationstates of plutonium. Another object of this phase of the invention is toprovide methods for oxidizing plutonium from a lower to a higher valencestate, and for reducing plutonium from a higher to a lower valencestate. A further object is to provide means for stabilizing lower andhigher oxidation states of plutonium in aqueous solutions of plutoniumions. Additional objects and advantages of this phase of the presentinvention will be evident from the following description.

In accordance with the present invention it has been found thatplutonium is chemically unlike osmium in many respects and is probably amember of a second rare earth group, the actinide series. It has furtherbeen discovered that plutonium, unlike a number of other members of thisseries, possesses a plurality of valence states. Plutonium has at leastfour valence states, including +3, +4, +5, and +6. In 0.5 M1.0 M aqueoushydrochloric acid the oxidation-reduction potentials are of thefollowing magnitudes:

As may be seen from the above couples, the stability of the higheroxidation states is dependent on the hydro- 'gen ion concentration. Inmoderately acidic solutions the Pu+ ion is generally very unstable anddisproportionates to Pu+ and Pu The Pu+ ion is capable ofdisproportionating to the Pu+ ion and the PuO ion, and in dilute aqueoushydrochloric acid this disproportionation may take place to aconsiderable extent. The Pu+ disproportionation is opposed, however, by

increase in hydrogen ion concentration and by the presence of ions whichtend to complex or otherwise stabilize the Pu+ ion. The effect ofadditional ions in hydrochloric acid solutions is illustrated by thefollowing potentials for the Pu+ Pucouple:

1.0 M HCl O.97 V. 1.0 M HCl--0.1 M H PO -0.80 v. 1.0 M HCl-LO M HF 0.53v.

Generally the anions of slightly ionized acids tend to complex the Pu+ion to a much greater extent than the anions of highly ionized acids.Thus, Pu+ is only slightly complexed by C10 3 Cl, and N0 1 it iscomplexed to a much greater extent by and it is very strongly complexedby PO F C H 0 and 0 0 In addition to the complexing effect of the anionsof the acids employed as solvents for plutonium, certain of these acidsmay also serve as oxidizing agents. However, at room temperatures, ormoderately elevated temperatures, and in the absence of oxidationcatalysts, the rate of oxidation by the acid is often so low that thiseffect may be ignored. Thus, the Pu+ ion is stable for considerableperiods of time in perchloric acid, although under proper conditions,the latter is capable of oxidizing Pu+ to PuO It is therefore desirableto control the state of oxidation of the plutonium by the use ofoxidizing agents and reducing agents which have rapid reaction ratesunder the conditions employed for processing the solutions.

The Pu ion may suitably be oxidized to the Pu0 ion by the addition of anactive oxidizing agent having an oxidation-reduction potentialsubstantially more negative than the oxidation-reduction potential ofthe Pu+ PuO couple in the particular solution employed. The followingare representative potentials for this couple:

1.0 M HCl 1.0 v. 1.0 M HNO -1.1v. 1.0 M H2804 1.3 V.

oxidizing agents having adequate oxidation-reduction potentials for usein such solutions may be chosen by reference to tables such as the tableof standard oxidation-reduction potentials given in the Reference Book 3of Inorganic Chemistry by Latimer and Hildebrand (The MacMillan Company,New York, 1934).

It is generally desirable to effect purification and concentration ofplutonium in nitric acid solutions. Examples of oxidizing agents for usein such solutions are bromates, permanganates, dichromates,silver-catalyzed peroxydisulfates, and ceric compounds. To effect theoxidation, a quantity of oxidizing agent at least equivalent to theamount of plutonium is added to the solution, and the resulting mixtureis digested at a moderately elevated temperature for a sufficient periodof time to insure complete oxidation of the plutonium. In most cases,this digestion may suitably be effected at 60-80 C. for 15-60 minutes.In order to maintain the plutonium in the hexavalent state forconsiderable periods of time after oxidation, it is desirable to employan excess of oxidizing agent to serve as a holding oxidant. This isespecially true if an acid solution is to be processed in ferrous metalequipment, or under other conditions permitting subsequent reduction ofthe plutonium.

Neptunium may be oxidized by any of the oxidizing agents mentionedabove, without the necessity of digestion at an elevated temperature.This greater rapidity of oxidation of neptunium at low temperatures maybe utilized to effect preferential oxidation of neptunium withoutsubstantial oxidation of plutonium. The preferred oxidizing agent forthis purpose is the bromate ion. At temperatures of 15-25 C. neptuniummay be substantially completely oxidized by alkali metal bromates innitric acid solutions, which contain ions such as 80.; ions to complex+4 plutonium, without appreciable oxidation of plutonium to theheXava-lent state. There is some evidence that bromate oxidation ofplutonium may be catalyzed by cerium, and it is therefore desirable toeffect the preferential oxidation of neptuniurn in ceriumfree solutions.

For the reduction of plutonium, reducing agents of adequate potentialmay be selected by reference to tables of standard potentials such asthe table previously referred to. The reduction may suitably be effectedby digestion at room temperature or slightly elevated temperatures.Digestion for 15 to 60 minutes at 15 to 35 C. will generally besatisfactory.

For the reduction of PuO or Pu+ to Pu, the reducing agent should have anoxidation-reduction potential substantially more positive than theoxidation-reduction potential of the Pu+ Pu+ couple in the solutionemployed. Thus, in 1.0 M HCl an active reducing agent having a potentialmore positive than 0.97 v. will be required, and in 1.0 M HNO;;, apotential more positive than 0.92 v. will be necessary. In order tomaintain the plutonium in the +3 valence state for appreciable periodsof time, it is desirable to maintain an excess of the reducing agent insolution.

In order to reduce PuO to Pu+ without reducing Pu+ to Pu, it isdesirable to employ an active reducing agent having anoxidation-reduction potential substantially more positive than theoxidation-reduction potential of the Pu+ PuO couple, and substantiallymore negative than the oxidation-reduction potential of the Pu+ Pu+couple, in the solution used. A wider selection of reducing agents ofthe desired potential will be available for use in solutions containingions which complex the Pu+ ion than are available for use in solutionswhich are substantially free from complexing effects. Thus, in 1.0 M HCland 1.0 M HCl1.0 M HF, the oxidation-reduction potentials areapproximately:

It may be seen that in the solution containing fluoride ion, reducingagents such as hydrogen peroxide and ferrous iron, which haveoxidation-reduction potentials of O.68 v. and .0.74 v. respectively,will reduce PuO only to Pu+ whereas in the solution without fluoride ionto complex the Pu+ ion, these reducing agents will tend to reduce theplutonium to the +3 state. A reducing agent, such as sulfur dioxide,having an oxidation-reduction potential of O.14 v. will tend to reducethe plutonium to the +3 state in either solution.

When employing the preferred solutions of plutonium in aqueous nitricacid, the reduction of PuO to Pu+ is preferably effected in the presenceof a complexing ion, employing reducing agents havingoxidation-reduction po tentials of the same magnitude as hydrogenperoxide and ferrous iron. However, it is also possible to use strongerreducing agents such as sulfur dioxide if any excess reducing agent isremoved or destroyed after the initial reduction is effected. In anycase, if Pu+ is desired, the hydrogen ion concentration should besufficiently high to oppose the disproprotionation of Pu+ to Pu+ and PuOFor this purpose, it is desirable to employ solutions having a pH notsubstantially above 2, and preferably considerably below 1. In the caseof aqueous nitric acid solution, it is generally desirable to maintain afree acid concentration of at least 1M.

It will be apparent that the considerations discussed above will alsoapply to the oxidation of Pu+ to Pu+ without oxidizing Pu+ to PuO by theuse of oxidizing agents having potentials intermediate the potentials ofthe two plutonium couples.

The plutonium oxidation and reduction processes described. above may beemployed, if desired, for the simultaneous oxidation or reduction ofboth neptunium and plutonium. Such simultaneous oxidation or reductionwill be attained provided equilibrium is reached. As previously pointedout, however, differential reaction rates may be utilized to attain oneoxidation state for neptunium and another oxidation state for plutonium.

The solutions of plutonium ions of the various valence states describedabove are useful for the electro-deposition of plutonium, for theprecipitation of plutonium compounds while leaving contaminatingcompounds in solution, and for the precipitation of contaminatingcompounds while leaving plutonium in solution.

The oxidation state of plutonium in aqueous solutions of the variousplutonium cations may be determined in accordance with methods commonlyused for the determination of the valence state of other metals insolution. Thus, the total plutonium in solution may be determined byquantitative gravimetric or radiometric analysis, and the percentage ofany particular ion may then be determined by a suitable differentialanalysis, such as quantitative oxidation or reduction, polarographicanalysis, or the like. Spectrophotometric analysis is especiallyadvantageous for determining qualitatively or quantitatively the variousplutonium ions in solution, in view of the sharp characteristic peaks inthe absorption spectra for the different valence states. Representativemolar extinction coefiicients for the Pu Pu, and PuO ions in aqueousinorganic acid solutions are given in the following tables:

Table 1 Pu+ in 1 M 1101 Wave length in A 4, 260 4, 560 4, 740 5, 050 5,620 6, 010 6, 660 8, 000 9, 090

Table '5-Continued Table v2 Pu 111 1 M HNo,

Weave length in 4, 040 4, 220 4, 480 4, 760 5, 020 5, 460 6, 600 7, 0808, 000 8, 550 Molar extinctlon eoetficient 27.0 24. 5 17.5 72. 5 8. 717. 31. 0 14.0 18. 9 13. 2

Table 3 Pu in 1 M H280 Wave length in A 4, 090 4, 360 4, 810 5, 480 6,640 7, 200 8, 140 8, 510 Molarextinetioncoefficient 29. 2 28. 5 85.220.0 39. 6 21.0 27. 1 14.3

Table 4 P1102+ in 1 M HNO;

Wave length in A 4, 590 4, 700 5, 060 5, 220 6, 240 8, 310 9, 580 9, 870Molarextinctioncoefficient. 15.0 14. 0 14.0 14.0 10.0 171.0 23.0 17.0

The suitability of various anions for the precipitation of insolubleplutonium compounds will depend on the oxidation state of the plutoniumand on the nature of the aqueous solvent from which the precipitation isto be made. I have found that the anions which are suitable for theprecipitation of an insoluble compound of hi valent or tetravalentplutonium fromany solution suitably comprise the anions which may beused to precipitate an insoluble compound of trivalent or tetravalentcerium from the same solvent. In the same manner that the solubility ofthe lower valence states of plutonium parallels that of cerium, thesolubility of hexavalent plutonium corresponds to that of hexavalenturanium.

The following plutonium compounds are insoluble in water, the terminsoluble being used to designate solubilities of less than 0.01 mol perliter:

It is generally desirable to precipitate plutonium compounds from acidicaqueous solutions, and especially from aqueous inorganic acid solutions.Representative solubilities of trivalent and tetravalent plutoniumcompounds in solutions of various acids and of various acidconcentrations are given in the following table:

Table 5 Trivalent plutonium Solubility Compound Aqueous solvent (mg. Pu/

liter) 1.0 M 69 1.0 M 49 1.0 M 37 0.2 M 25 0.6 M 7 0.6 M 28 0.8 M 20 0.8M 66 0.8 M 210 0.8 M 23 0.8 M 120 0.8 M 900 Tetravalent plutoniumSolubility 5 Compound Aqueous solvent (mg. Pu/

liter) Fluoride 0.1-2.0 M HNO3+0.52.0 M HF- 350-700 Ptgassum double0.52.0 M HNO;+0.5-2.0 M HF. 10-50 mm e. Lanthanum 0.5-2.0 M HNO;+0.52.0M HF- 20-70 10 double fluoride.

date 24 84 73 103 4 11 15 47 95 32 15 11 a 14 25 20 29 1 i 25 23 I 4 63550 12 16 29 30 72 710 HNOa+3.1% H202 70 l. HNO3+5.4% H20 50 1.0 MHNO3+7.5% H202".-- 20 0.01 M Na2SO4+HrSO4, pH 5.4. 0. 02 0.01 MNazs04+H2sO pH 4 1. O. 2 0.01 M Na2SO4+H2S04, pH 3.7- 0. 5 0.01 MN82SO4+H2SO4, pH 3.0. 1. 2 0.01 M NazSO4-i-HzSO4, pH 2.5-. 20. 9 1.0 MN3ZSO4+H2SO4, H 6.3---. 1.1 1.0 M Na2SO4+HzS04, D 5 1 l 16 1.0 MNaZSO4-I-HQSO4, pH 4.2 217 40 1.0 M N ZSOA-I-H2SO4: PH 3.6.... 300 1.0 MNa2SO4+HzSO4, pH 3.2 355 0.1 M NaClO4+HO104, pH 1.3.- 8. 0 0.1 M NaClO4+HOlO4, pH 1.6- 4. 2 0.1 M N aOlO4+HGlO4, pH 1.8 2.1 0.1 MNaO10l+HOlOo P 2.2. 1. 0 0.1 M NaO1O4+HOlO4, pH 3.1- 0. 4 0.1 MNaOlO4+HOlO4, pH 4.1- 0.3 0.1 M NaO1O4+H0lO4, pH 5.0 0. 2

Anion Cations which form soluble compounds Fluoride Cs, Rb, Zr, Nb-+ Ag.Orthophosphate Cs, Rb. Iodate Cs; Rb, La, Coi and other rare earths.

The reduction of plutonium is illustrated in the following example byway of its preoipitationon a carrier.

EXAMPLE 1 7 An 8.6 N sulfuric acid solution was prepared containinglanthanum sulfate in a concentration of approximately 430 mg. per literand plutonium in tracer concentration. To this solution was added about2.1 times its volume of a saturated aqueous solution of sulfur dioxide,and the mixture was allowed to stand at room temperature for 25 minutesto effect reduction of any hexavalent plutonium. The sulfuric acidconcentration of the resulting solution was about 2.8 N and thelanthanum sulfate concentration was about 139 mg. per liter. About 27%by volume of 48% aqueous hydrofluoric acid was then added to thesolution and the resulting lanthanum fluoride precipitate was separatedby centrifuging. Analyses for alpha radiation showed that theprecipitate contained 93% of the plutonium which was present in theoriginal solution.

The following example illustrates the direct precipitation of aninsoluble plutonium compound, without a carrier, from a solution derivedfrom a preceding carrier precipitate after reduction of the plutonium bythe process of this invention.

EXAMPLE 2 A mixture of hydroxides comprising 88.2% by weight oflanthanum hydroxide, 9.9% plutonium hydroxide and 1.9% potassiumhydroxide, was dissolved in 2.03 times its Weight of 16 N nitric acid.Approximately 3.22% by weight of concentrated sulfuric acid (sp. gr.1.84) was added to the resulting solution, which was then diluted withwater to form a solution 0.8 N with respect to nitric acid and 0.2 Nwith respect to sulfuric acid. The plutonium concentration of thissolution was 8.25 g. per liter. The solution was heated to 60 C. and 50%by volume of 30% aqueous hydrogen peroxide was added over a period ofone hour. The resulting slurry was digmted for an additional hour atroom temperature, after which the plutonium peroxide (probably a basicperoxidic sulfate) was separated by filtration. The precipitate was thendissolved in 16 N nitric acid, sulfuric acid was added, and the solutiondiluted to 0.8 N HNO -0.2 N H SO Plutonium peroxide was thenreprecipitated and separated by filtration as before. The reprecipitatedproduct, which was free from lanthanum, amounted to 99% of the plutoniumoriginally present in the lanthanum hydroxide mixture.

In order to remove fission products by precipitation for the purpose ofdecontaminating plutonium, the plutonium is maintained in solution inthe hexavalent state while precipitating a carrier for the contaminatingcations. The carrier for this procedure may suitably comprise a carrierfor trivalent or tetravalent plutonium, and such a carrier is highlyadvantageous when employed alternatively as a carrier for reducedplutonium and as a carrier for contaminants from a solution containingoxidized plutonium. In such a combination procedure, contaminants whichwere carried with reduced plutonium in one step of the process may becarried away from oxidized plutonium in a succeeding step employing thesame carrier. Conversely, contaminants which would be carried withreduced plutonium on a given carrier may first be carried, on thatcarrier, away from oxidized plutonium.

The following example illustrates decontamination by anoxidation-reduction carrier cycle:

EXAMPLE 3 Plutonium was separated from the uranium and fission productscontained in uranyl nitrate hexahydrate which had received 100milliampere hours of neutron bombardment. The uranyl nitrate had beenstored for approximately four weeks after bombardment, and it containeda substantial amount of 94 Pu but was practically free from 93 N The 94Pu was separated by the following procedure:

Approximately 1053 parts by weight of the bombarded uranyl nitratehexahydrate described above and approximately 30 parts by weight ofthorium nitrate dodecahydrate were dissolved in suflicient nitric acidto produce a 8 solution 2 N with respect to nitric acid after theaddition of 3186 parts by weight of a 0.35 M potassium iodate solution.incorporated as a tracer to give an a count of 10,000 per minute per m1.of the final mixture. The potassium iodate solution was then added,producing a solution having a uranium concentration of approximately0.050 g. U per ml. This solution, containing the resulting thori umiodate precipitate, was allowed to stand for twenty minutes at roomtemperature.

The thorium iodate precipitate, containing the bulkofthe plutonium, wasthen filtered off and washed with a solution 1.0 M with respect tonitric acid and 0.1 M with respect to potassium iodate. The washedprecipitate was dissolved in 1188 parts by weight of 12 N hydrochloricacid, 2198 parts by weight of 0.5 M sodium dichromate solution wasadded, and the resulting solution wasthen diluted with water to aconcentration 2.4 N -with respect to hydrochloric acid and 0.1 M withrespect to sodium dichromate. This solution was then digested for onehalf-hour at 65 C. to eliect oxidation of the Pu+ to Pu+ The Pu+solution was then cooled to room temperature, after which 4248 parts byweight of 0.35 Mpotassium iodate solution was added, and the mixture wasallowed to stand for twenty minutes at room temperature. The resultingthorium iodate precipitate, containing the bulk of the fission products,was filtered ofi and washed in the same manner as the first thoriumiodate precipitate.

The distribution of the plutonium and fission products between the firstthorium iodate precipitate, the first supernatant liquid, the secondthorium iodate precipitate, and the second supernatant liquid, wasdeterminedby measurement of the on, p, and '7 radiation omitted. Forthis purpose, blank determinations were first made on the originalmixture, prior to the separation of the first thorium iodateprecipitate. The oz count on this original mixture was taken to be thatof the added 94 Pu Aliquots were analyzed for total {3 count, and fortotal [3 count corrected for the UX p count, by means of Geiger-Muellercounters and well known techniques. Aliquots of the two thorium iodateprecipitates and of the two supernatant liquids were then analyzed for 5and 7 activities in the same manner.

The plutonium content of the two thorium iodate precipitates and of thetwo supernatant liquids was recovered by an additional precipitation ineach case, and the or activity of each of the precipitates was thendetermined. Since 94 Pu and 94 Pu have identical chemical properties,the distribution of 94 Pu as indicated by the a counts, also representedthe distribution of the 94 Pu The distribution of uranium, plutonium,and fission products obtained by the above separation procedure is shownin the following table:

Table 6 Percentage of original substance The following exampleillustrates concentration of plutonium with respect to its carrier, aswell as decontamination, in an oxidation-reduction carrier cycle:

EXAMPLE 4 Lanthanum fluoride, carrying plutonium as the onlyalpha-active component, and carrying beta-activecon Sufiicientradioactive plutonium, 94 Pu ,--was- 9 tamifiants, was dissolved in amixture of nitric and sulfuric acids. The solution was evaporated untilfumes of sulfur trioxide were evolved and was then cooled and dilutedwith water to 30 times the volume of the fuming 10 which differs fromthe plutonium carrier as to both cation and anion. I II a A t I EXAMPLESa A cerous fluoride precipitate carrying plutonium as solution Arnixtureof potassium peroxydisulfate and theIonlyvalphamcfivecqmponentfland cmyingvbetaacfive sllver filtrate a F of 20 to was thenadded contaminants, was subjected to radioactive analysis for thesolution was d gested for 15 minutes toeffect oxidatotal alpha and betaradiation The precipitate was Q Plutomum to hexavalent f Hydro solved innitric and sulfuricacids, the solution evaporated fluoric acid Was thenadded m a concentration in excess I to drynessi and the residuedissolved in aqueous nitric of the equivalent concentration of lanthiumion. After An excess of potassium bromatewas introduced digestion for 5minutes the lanthanum fluoride precipitate and the bmmate ion, catalyzedby cerium, Ioxidized was P P y centll-fugmgplutonium from thetetravalent to the hexavalent state, The centrifugate was evaporateduntil fumes of sulfur A substantial opo ti nn f the Camus ion was SimulgtIlOXidG Were evolved, thus destroying the PfiIOXYdlSLlltaneouslyoxidized to eric i011, Thorium i011 and an fate and eifecting reductionof the plutonium to the tetraexcess of iodate ion-were then introducedto precipitate valent state, and the solution was then cooled anddiluted mixed thorium and ceric iodates. This precipitate was withwater. Lanthanum ion, an amount less than that in separated I bycentrifuging and was dissolved and reprethe preceding precipitate,together with an excess of hycipi d addlflonal ihorlulu f G Q tdrofluoric acid, were then introduced. The resulting Ian- 20ThBICeIItYIfUgatBISIfI' 01.11 the two iodate P eclpltatiis were thanumfluoride precipitate was separated by centrifug- QP and evaporated W1thooncentrated hydrochlonf ing, washed with dilute hydrofluoric acid, anddried. q resultmg m W5 cooled, and The ratios of plutonium to lanthanumfluoride carrier g g g i i g f i 3 g a in the initial material and inthe final precipitate were deg q W i e 3 {3 e igg termined on the basisof alpha radiation and weight of F 5 non preplp 6 W5 separ y eel} nlanthanum It was found that the ratio of lutonium washed wlth dllutehydrofluonc acid and dned t I th fin 1 t 131 Radioactive analysis of theiodate precipitates showed g i i e .2 prficlpl a f 6 Ta them to beinactive with respect to alpha radiation, thus i mm a W ereas f f eta iindicating no by-product loss of plutonium. Analysis mation to carrier to the final preclpltate as only 13% of the final plutonium-carryingcerous fluoride, precipitate of the ratio in the imtial material. I Ishowed it to contain only one third of the beta radia: e fqllowmg ampIllustrates an a p titm of the initial cerous fluoride precipitate. troncarrier cycle utilizing smiultaneous precipitation of Combinations ofvariousreducing and oxidizingagents two contarmnant carriers, one ofwhich contains the same as applied to diiferent carrier precipitationsare set forth cation element as the plutonium carrier and the other ofin Tables 7 and 8.

Table 7 Preceding First pluto- Matathe- Fission prod- Reducing Secondpluto- Metathev Final solution nium carrier sizing Subsequent solutionoxidizing agent not carrier agent niurn carrier sizing solutionprecipitate agent precipitate precipitate agent (ZIO)3(PO4)2 Hi0i(ZIO)H(PO4)2 Aq HNot LaFs HzCzOi U(C204)2 Aq HNO; LeFt+oeFi so o H 1.LEKCZOOa-F LaF Th(IOa)4 L8.2(C1O4)3 Ce(IO Aq GEE; UO4.XHZO Aq H01. CGFH102"... LaFa NaOH Aq HNOs ThE Laz(C204)3 Aq HNOa T11F4 (ZIO)3(PO4)7 AqHNOQ oet(P0i)i. ThF4 NaOH Aq H01. (ZlO)a(PO4)2. LaFa (Zr0)3(Poi)i. NaOH(ZI'O)3(PO4)Z Table 8 First pluto- Metathe- First fission Secondi'ission n v Second pluto- Metathee Preceding niurn carrier sizingSubsequent Oxidizing product product Reducing nium carrier sizing Finalsolution precipitate agent solution agent carrier carrier agentprecipitate agent solution precipitate precipitate I e A nNot A HNO3KMHO4 Lari (ZI'O)3(PO4)2. H2o2- (ZIO)3(PO4)2 Aqfirioi Aq.HOl Aq.HO1OB(NOa)4 LaF; CeFi B20204- C1(Cz04)a AQ.HNO3 Aq. HNOs.. Kfaszgfta- LaFs20 Th04.XHzO

g AQ-HNOs Aq.(NH4)2CzO4 1194(02093--- La(OH)3 Aq.HC1. Aq.HNO3 LaP THi0i. Th(IO3)4 Aq.HNO Aq. HN03 s02 UO4.XH20- AQ.HN03 Aq. HNO3 sot-.. TumAq. HCL- Cez(C20i)a Aq. HSIOH- 112C204- T (CRO-l)fl. Aq.HC1 Thump"... 0'Kiortotmq zr0)'t(P0i)t. H20204. Aq. HO Th(IO )4 Kioriotm. L3PO4 20 m.Aq.HNOa ThF4. K ig ia- I C6(I03)4. s0i... o

g v Aq. HOL. U(C204)2 Ar I lNgfi Kioriotim (ZlO)3(PO4)2- OeFt.. soi,

I Aq.HNOs tzrontroni 2 2 7 mod... zto)t Poi i s0i Aq. H01-.. ZIO(IO3)2-K2C12O7 Z1O(IO )2 11202.... Aq.HO1- ZIO(IO3)Z Kfig g- LaFi Hi0i.

- In the recovery of plutoniumfrom neutron irradiated uranium by thedecontamination and concentrationprocedures previously described, aprecipitate may finally be obtained which consists of a single carrierand a substantially pure plutonium compound having the same anion as thecarrier. Such a precipitate desirably has a low carrierto-plutoniumratio, e.g. 100/1 or lower. When a precipitate of this character isdissolved in a minimum quantity'of an aqueous inorganic acid,substantially pure oxygenated compounds of plutonium may be precipitatedfrom the resulting solution.

The term oxygenated compound of plutonium, as used herein and in theappended claims, signifies a compound having at least one oxygen atomdirectly bonded to .a plutonium atom. Plutonium peroxide and'the variousbasic peroxidic salts of'tetravalent plutonium are examples of plutoniumcompounds having directly bound oxygen. These compoundsmay beprecipitated from acidic solutions of tetravalent or hexavalentplutonium by the addition of a suitable peroxide, preferably hydrogenperoxide. The resulting hydratedprecipitattes are commonly mixtures ofcompounds having different ratios of oxy groups, peroxy groups, and acidanions, with the result that the over-all ratios are generallynon-integral. Representative products of this class are shown below:

2.54(c1) 0.45 ('0=) 0.482.297. 120Pu(O")2.61(SO4=)O.14(N0a)O.19(O=)0.46-1.65H2O Such products may containhydrated compounds of the following types:

The following example illustrates the preparation of a basic peroxidicplutonium nitrate-sulfate:

EXAMPLE 6 A lanthanum fluoride-plutonium fluoride precipitate containingabout 25% by weight of plutonium tetrafluoride is fumed withconcentrated sulfuric acid until no further hydrogen fluoride isevolved. The material is then dissolved in aqueous nitric acid to form asolution 1.0 N with respect to nitric acid and 0.1 M with respect tosulfuric acid, having a lanthanum concentration of about 37.1 g. perliter, and a plutonium concentration of about 13.2 g. per liter. Aqueoushydrogen peroxide (30% H by weight) is then added over a period of onehour, at 20 C., in an amount such that the final solution contains 10% H0 by weight. The slurry is then digested for one hour at 20 C. andfiltered. The product thus obtained is a blue-green solid correspondingto an empirical formula It is readily soluble in acids and is useful forthe preparation of other plutonium compounds.

Plutonium hydroxides and the various basic salts of tetravelentplutonium are additional examples of plu- 1'2 tonium compounds havingdirectly boundoxygen. Compounds of this class may be precipitated byneutralizing acidic solutions of trivalent or tetravelent plutonium. Theresulting products are obtained as hydrated precipitates comprisingmixtures of different compounds such that the over-all ratio ofhydroxide ion to acid anion is usually non-integral. Such mixtures maybe represented by empirical formulas such as If such precipitates aredried without washing, Partial dehydration of the hydroxide, ortransformation toa hydrated oxide structure may occur, and the mixturesmay then be represented by empirical formulas such as 2( s)x' 2 and PuO(SO -yH 0 where x and y may be, but usually will notbe, integers.

Such products derived from tetravalent plutonium may contain compoundsof the following types:

If an initial precipitate of the type described above is thoroughlywashed or digested in alkali, substantially pure hydroxide is obtained.On drying the resulting hydroxide, even at moderately elevatedtemperatures, it is transformed at least partially to the hydratedoxide. The following example illustrates a suitable procedure for thepreparation of this compound:

EXAMPLE 7 A basic peroxidic plutonium nitrate-sulfate precipitate,prepared as in Example 6, is dissolved in 16 N nitric acid *and dilutedwith water to form a solution having a nitric This compound is solublein aqueous solutions having a pH of at least 7.0 to the extent of lessthan 2 mg. of plutonium per liter. It is readily soluble in strong acidsand is useful for the preparation of other plutonium compounds.

The procedure of the above example may be modified by saturating thesolution with sulfur dioxide prior to neutralizing with ammoniumhydroxide. In such case the product obtained is the trivalent plutoniumhydroxide. This compound is a blue solid which is readily soluble inacids and soluble in 5 N ammonium hydroxide to the extent of about 0.09g. of plutonium per liter. This compound, as well as the tetravalenthydroxide is particularly useful for the production of other plutoniumcompounds.

Ignition of the hydrated oxides of plutonium results in completedehydration to the corresponding oxides. Partial dehydration takes placeat temperatures only slightly above room temperature, but it isdesirable to heat the material to a temperature in the range of 500-1000 C. to effect complete dehydration. The following exampleillustrates the production of plutonium dioxide.

EXAMPLE 8 Tetravalent plutonium hydroxide prepared as in Example 6, isignited to constant weight in a mufiie furnace at about 850 C. Theresulting compound, P is a crystalline solid appearing green byreflected light and yellow by transmitted light. It is soluble in strongmineral acids. The crystalline structure is face-centered cubic, withfour molecules per unit cell and a lattice constant of 5.386:0.002A. Thecalculated density is 11.44.

Plutonium dioxide may also be prepared by the ignition of the plutoniumnitrates or the basic peroxidic plutonium nitrates at temperatures above300 C., and preferably at a temperature in the range of 500-1000 C.

The various soluble salts of plutonium may be prepared by dissolvingplutonium hydroxide in the acid having the desired cation, adjusting theoxidation state of the plutonium in the resulting solution, andevaporating to crystallize out the desired compound. The insoluble saltsmay be prepared in a similar manner by dissolving plutonium hydroxide inan acid which forms a soluble salt, adjusting the oxidation state of theplutonium, incorporating the cation of the desired insoluble salt, andseparating the resulting precipitate. The following examples illustratethe preparation of representative plutonium salts:

EXAMPLE 9 Tetravalent plutonium hydroxide is dissolved in 16 N nitricacid and the resulting solution is evaporated until the tetranitratecrystallizes out. The product is obtained as a highly hydrated lemonyellow crystalline material corresponding to the formula Pu(NO -xH O.This compound is very soluble in water and in dilute acids. Aconcentration as high as 2.5 M in 1.7 N nitric acid is obtainable,although this solution may be somewhat super-saturated at roomtemperature.

EXAMPLE 10 Tetravalent plutonium hydroxide is dissolved in 16 N nitricacid, diluted to l N nitric acid, and heated to 75- 100 C. untilspectrophotometric analysis indicates complete oxidation of theplutonium to the hexavalent state. The solution is then cooled andconcentrated by evaporation under high vacuum until plutonyl nitratecrystallizes out. The product is yellow to orange in color, with a pinktinge, and corresponds to the formula It is extremely soluble in water(ca. 500 g. Pu per liter). Treatment with various organic solventsresults in solution of PuO (N in the organic phase.

EXAMPLE 11 Trivalent plutonium hydroxide is dissolved in concentratedhydrochloric acid and the resulting solution is evaporated in a streamof hydrogen chloride until PuCl -6H O crystallizes out as a blue solid.This compound is deliquescent and melts at 94-96 C. It may be dehydratedin vacuo (less than -1'mrn. Hg pressure) at room temperature to themonohydrate, PuCl -H O. The monohydrate may be slowly transformed toan-' hydrous PuCl at 70 C. in a high vacuum (10 mm. Hg pressure). Theanhydrous trichloride is more conveniently prepared, however, by heatingthe hexahydrate slowly to 250 C. in a stream of hydrogen chloride.

PuCl is a blue-green colored solid which is readily soluble in water toform purple colored solutions. The solid is stable with respect to airoxidation at room temperature but is converted to PuO on heating in airto 400 C. The crystalline structure of the trichloride is hexagonal withtwo molecules per unit cell. The lattice constants are a =7.38Oi0.00l A.and a =4.238i-O.00l A.

and the calculated density is 5.70.

14 EXAMPLE 12 The procedure of Example 11 is modified by substitutinghydrobromic acid for hydrochloric acid and substituting hydrogen bromidefor hydrogen chloride in the,

drying operation. The final product, anhydrous plutonium tribromide, isa blue-green crystalline compound having the following properties:

Melting Point-654 0.:4". Crystalline Structure:

orthorhombic. Four molecules per unitcell. Lattice constants: a =12.57-0.0 5 A. a =4.11- -0.03 A. a =9.13i0.04 A.

Calculated density6.69.

EXAMPLE 13 The calculated density is 4.89. E

The hydrated tetrafluoride may be dehydrated at 350 C. in a stream ofhydrogen fluoride to yield the anhydrous compound. PuF is a yellow topale brown crystalline compound which is soluble in hot concentratedsulfuric acid or nitric acid. The crystalline structure is mono! clinicwith 12 molecules per unit cell. The lattice constants are: A

The calculated density is 7.0.

EXAMPLE 14 The procedure of Example 13 is modified by saturating thehydrochloric acid solution with sulfur dioxide prior to incorporatingthe hydrofluoric acid. The resulting precipitate is plutoniumtrifluoride, which is separated from the supernatant solution and driedat 300 C. in a stream of hydrogen fluoride. The resulting product isanhydrous PuF a crystalline solid of purple toblack color having "amelting point of 1141 C.i7. The crystalline structure is hexagonal withtwo molecules per unit cell. The lattice constants are:

a =4.087:0.001 I A. a =7.240i0.001 A.

The calculated density is 9.32. 7 7 l Plutonium trifluoride may bedissolved by fuming with I EXAMPLE 15 Tetravalent plutonium hydroxide isdissolved in 16 N nitric acid and the resulting solution is diluted toabout 2 N HNO Potassium nitrate is then added in excess of theequivalent plutonium nitrate concentration and a potassium-plutoniumdouble fluoride is precipitated by the addition of hydrofluoric acid.The composition 1 d of the final solution from which the precipitationis made is approximately 1.8 N HNO 0.5 N KNO -3.0 N HF The precipitateis a pale green material corresponding to the empirical formula KPuF -xHO. After drying in vacuo at room temperature the product is obtained asan anhydrous crystalline compound conforming to the formula KPuF Thecrystalline structure is rombohedral with six molecules per unit cell.The lattice constants are:

Other double fluorides may be prepared by substituting other alkalimetals or ammonia for the potassium in the above example.

EXAMPLE 16 solution is approximately 14.6 mg. per liter.

EXAMPLE 17 Tetravalent plutonium hydroxide is dissolved in 16 N nitricacid and the resulting solution is diluted nearly to 1.0 M HNOOrthophosphoric acid is then added in an amount sufiicient to produce asolution. The resulting white precipitate after drying in vacuo at roomtemperature conforms to the empirical formula This compound isisomorphic with the corresponding ceric and thorium phosphates. It issoluble in 0.6 M H PO -1.O M HNO;; to the extent of about 19 mg. Pu perliter.

EXAMPLE 18 The procedure of Example 17 is modified by saturating thenitric acid solution with sulfur dioxide prior to incorporating thephosphoric acid. In this case the dried precipitate is a violet coloredcrystalline compound which conforms to the empirical formula PuPO -xH O.It is soluble in 0.6 M H3PO4-LO M HNO solution to the extent of about 28mg. Pu per liter. The crystalline structure of this compound ishexagonal, with three molecules per unit cell. The lattice constantsare:

The calculated density is 6.04.

EXAMPLE 19 Tetravalent plutonium hydroxide is dissolved in aqueoussulfuric acid to form .a solution approximately 2 M with respect to H 50and approximately 0.4 M with respect to Pu(SO About 40 percent by volumeof methyl alcohol is added to the sulfuric acid solution and the mixtureis allowed to stand for 16 hours. Approximately 7 percent by volume ofconcentrated sulfuric acid is then added, with agitation, and theprecipitate is separated from the supernatant solution. The product is apink crystal- 16 line solid which on analysis was found to conform tothe formula PU(SO4)24H20.

Metallic plutonium may be produced by reduction of any of the plutoniumhalides with active metals such as the alkali and alkaline earth metals.

Metallic plutonium and the various plutonium compounds which have beendescribed above are extremely useful for the production of atomicenergy. Pu in the metallic state, or in the form of any of the compoundspreviously described, can undergo nuclear fission in a selfsustainingneutronic chain reaction. The critical size of a single mass ofplutonium metal for self-sustaining chain.

reaction is of the order of 10 kg. The critical size of a single mass offused plutonium trichloride is of the order of 25 kg., and the criticalsizes for the other. compounds described above will lie between that ofthe metal and that of the itrichloride. When the plutonium or plutoniumcompound is dispersed in a neutron-slowing material, termed a moderator,the critical mass for self-sustaining chain reaction becomes very muchless than. for the pure material. Under optimum conditions, the criticalmass of plutonium may be as low as 200 grams. This quantity of plutoniumin a single mass of material should not be exceeded Without providingadequate neutron-absorbing safety devices.

Plutonium or mixtures of plutonium and other fissionable isotopes may beutilized for the production of atomic energy in neutronic reactors inaccordance with the disclosure of copending application Serial No.634,311, filed December 11, 1945, by Emilio Segre, Joseph W. Kennedy,and Glenn T. Seaborg and granted as US. Patent No. 2,908,621 on October13, 1959. In such utilization, metallic plutonium may be dispersed in asolid moderator such as graphite in a lattice structure, or any of theplutonium compounds described above may be employed as solutions ordispersions in a liquid moderator such as deuterium oxide.

Metallic Pu and Pu and all of the compounds of these isotopes are alsouseful as sources of alpha radiation. In conjunction with ot-n reactingelements such as beryllium, they may also be employed as neutronsources.

It is to be understood that this aspect of the present invention is notlimited to the specific compounds and methods of preparation which havebeen described above by way of illustration. Other analogous compoundsand equivalent methods of preparation are included inthe scope of thisphase of the invention. Also, the particu lar oxidizing and reducingagents, processes, and solutions discussed herein are merelyillustrative and are not to be construed as limiting the scope of thepresent invention. Other oxidizing and reducing agents having therequired potentials may be utilized instead of those specificallymentioned, and the procedures may be modified in numerous respects, aswill be evident to those skilled in the art.

What is claimed is:

1. A process for controlling the oxidation state of plutonium in anaqueous solution containing plutonium ions, which comprisesincorporating in said solution an agent of the class consisting ofoxidizing agents selected from the group consisting of bromate,permanganate, ceric ions, dichromate and peroxydisulfate plus silvercation and reducing agents selected from the group consisting ofhydrogen peroxide, ferrous ions, sulfite ionsand sulfur dioxide.

2. A process for oxidizing plutonium from a lower oxidation state to ahigher oxidation state in an aqueous inorganic acid solution containingplutonium ions, which comprises incorporating in said solution anoxidizing agent selected from the group consisting of bromate,permanganate, ceric ions, dichromate and peroxydisulfate plus silvercation and digesting the resulting mixture at a moderately elevatedtemperature until the oxidation reaction is substantially complete.

3. A process of oxidizing plutonium from an oxidation state not greaterthan +4 to an oxidation state of +6 in an aqueous nitric acid solutioncontaining plutonium ions, which comprises incorporating in saidsolution an oxidizing agent selected from the group consisting ofbromate, permanganate, ceric ions, dichromate and peroxydisulfate plussilver cation and digesting the resulting mixture at a temperature of 60to 80 C. for 15 to 45 minutes.

4. The process of claim 3 in which the oxidizing agent is an alkalimetal dichromate.

5. The process of claim 3 in which the oxidizing agent is a silvercatalyzed alkali metal peroxydisulphate.

6. A process for reducing plutonium from a higher oxidation state to alower oxidation state in an aqueous inorganic acid solution containingplutonium ions, which comprises incorporating in said solution areducing agent selected from the group consisting of hydrogen peroxide,ferrous ions, sulfite ions and sulfur dioxide and digesting theresulting mixture at a moderately elevated temperature until thereduction reaction is substantially complete.

7. A process for reducing plutonium from an oxidizing state of +6 to anoxidation state not greater than +4 in an aqueous nitric acid solutioncontaining plutonium ions, which comprises incorporating in saidsolution a reducing agent selected from the group consisting of hydrogenperoxide, ferrous ions, sulfite ions and sulfur dioxide and digestingthe resulting mixture at to 35 C. for 15 to 60 minutes.

8. The process of claim 7 in which the reducing agent comprises thesulphite ion.

9. The process of claim 7 in which the reducing agent is incorporated bysaturating the solution with sulphur dioxide.

10. A process for controlling the oxidation state of plutonium in anaqueous solution containing plutonium ions, which comprises providing insaid solution suflicient oxidizing agent selected from the groupconsisting of bromate, permanganate, ceric ions, dichromate andperoxydisulfate plus silver cation to maintain the plutonium in anoxidation state of +6.

11. A process for controlling the oxidation state of plutonium in anaqueous solution containing plutonium ions, which comprisesincorporating in said solution a suflicient amount of a reducing agentselected from the group consisting of hydrogen peroxide, ferrous ions,sulfite ions and sulfur dioxide to maintain the plutonium in anoxidation state of +3.

12. A process for controlling the oxidation state of plutonium in anaqueous solution containing plutonium ions, which comprises reducing anyhexavalent plutonium in said solution to an oxidation state not greaterthan +4 with a reducing agent selected from the group consisting ofhydrogen peroxide, ferrous ions, sulfite ions and sulfur dioxide,maintaining said solution free from excess reducing agent, and providingin said solution an anion which complexes tetravalent plutonium, saidanion being selected from the group consisting of fluoride anion,acetate anion, oxalate anion and sulfate anion.

13. The process of claim 12 in which the anion is the sulphate ion.

14. The process of claim 12 in which the anion is the fluoride ion.

15. A composition of matter consisting essentially of an aqueoussolution containing trivalent plutonium ions.

16. A composition of matter consisting essentially of an aqueoussolution containing tetravalent plutonium ions.

17. A composition of matter consisting essentially of an aqueoussolution containing ions of hexavalent plutonium.

18. A composition of matter consisting essentially of an aqueoussolution containing tetravalent plutonium ions and anions of anincompletely ionized acid selected from 18 the group consisting ofsulfate, fluoride, acetate and oxalate anions.

19. A composition of matter consisting essentially of an aqueoussolution containing tetravalent plutonium ions and sulphate ions.

20. A composition of matter consisting essentially of an aqueoussolution containing tetravalent plutonium ions and fluoride ions.

21. A composition of matter consisting essentially of an aqueoussolution containing hexavalent plutonium ions and an oxidizing agentselected from the group consisting of bromate, permanganate, ceric ions,dichromate and peroxydisulfate plus silver cation.

22. A composition of matter consisting essentially of an aqueoussolution containing hexavalent plutonium ions and dichromate ions.

23. A composition of matter consisting essentially of an aqueoussolution containing hexavalent plutonium ions, silver ions, andperoxydisulphate ions.

24. A composition of matter consisting essentially of an aqueoussolution containing trivalent plutonium ions and a reducing agentselected from the group consisting of hydrogen peroxide, ferrous ions,sulfite ions. and sulfur dioxide.

25. A composition of matter consisting essentially of an aqueoussolution containing plutonium ions and sulphite ions.

26. A composition of matter consisting essentially of an oxide ofplutonium haw'ng the formula PuO 27. A composition of matter comprisingplutonium dioxide, having the empirical formula PuO' 28. A compositionof matter consisting essentially of a basic peroxidic compound ofplutonium of the formula P11207- 29. A composition of matter consistingessentially of a basic peroxidic plutonium nitrate having the formula 2e( s)2- 30. A composition of matter consisting essentially of aplutonium salt of an inorganic acid having the formula 2 5( s)2- 31. Acomposition of matter consisting essentially of a plutonium salt of aninorganic oxy acid having the formula PH(OH)2(NO3)2.

32. A composition of matter consisting essentially of a nitrate ofplutonium having the formula Pu( OH) N0 33. A composition of mattercomprising a nitrate of tetravalent plutonium having the empiricalformula PI1(NO3)4'JCH20.

34. A composition of matter comprising plutonyl nitrate having theempirical formula PuO (NO -xH O.

35. A composition of matter comprising tetravalent plutonium iodatehaving the empirical formula Pu(IO 36. A composition of matterconsisting essentially of a phosphate of plutonium having the formulaPuPO 37. A composition of matter comprising tetravalent plutoniumorthophosphate having the empirical formula 3( 4)4- 38. A composition ofmatter consisting essentially of a sulfate of plutonium having theformula PuO SO 39. A composition of matter comprising tetravalentplutonium sulfate having the empirical formula Pu(SO 40. A compositionof matter consisting essentially of a halide of plutonium having theformula PuX wherein X is a halogen selected from the group consisting offluorine, chlorine and bromine.

41. A composition of matter comprising trivalent plutonium chloridehaving the empirical formula =PuCl 42. A composition of mattercomprising trivalent plutonium fluoride having the empirical formula PuF43. A composition of matter comprising tetravalent plutonium fluoridehaving the empirical formula PuF 44. A composition of matter comprisingtrivalent plutonium bromide having the empirical formula PuBr(References on following page) References Cited in 3h? file of thispatent UNITED STATES PATENTS Thompson et a1. Mar. 19, 1957 OTHERREFERENCES Seaborg et al.: The Actinide Elements, pages 371- 433;published by McGraW-Hill Book Co., N.Y. (1954), Article by Cunningham.

Seaborg et al.: The Transuranium Element. 'Published by the McGraw-HillBook Co., N.Y. (1954).

Articles by: Hamaker et al., pages670, 671, 673, 674 and 681; Abraham etal., pages 741, 757; Anderson, pages 796-800; Mooney et al., page 442;Zachariasen (I), pages 1463 and 1464; Zachariasen (II), pages 1477 and1485.

Seaborg et al.: The Actinide Elements, pages 250. Published byMcGraW-Hill Book Co., N.Y., 1954.

Seaborg et al.: The Transuranium Elements. Published by McGraw-Hill BookCo., N.Y., 1949 pages 25- 38 (Article prepared March 19, 1942).

1. A PROCESS FOR CONTROLLING THE OXIDATION STATE OF PLUTONIUM IN ANAQUEOUS SOLUTION CONTAINING PLUTONIUM IONS, WHICH COMPRISESINCORPORATING IN SAID SOLUTION AN AGENT OF THE CLASS CONSISTING OFOXIDIZING AGENTS SELECTED FROM THE GROUP CONSISTING OF BROMATE,PERMANGANATE, CERIC IONS, DICHROMATE AND PEROXYDISULFATE PLUS SILVERCATION AND REDUCING AGENTS SELECTED FROM THE GROUP CONSISTING OFHYDROGEN PEROXIDE, FERROUS IONS, SULFITE IONS AND SULFUR DIOXIDE.